This paper proposes a concept of “lower temperature and higher pressure combustion” for natural gas engines in order to simultaneously achieve high performance, high reliability and low emissions. This concept should not only improve engine performance but also reduce engine thermal load (improve reliability) by adopting low engine speed specifications with the Miller cycle or EGR system while maintaining power output.This paper experimentally examines the effects of engine speed on performance, such as engine efficiency, friction loss, pump loss, heat loss, exhaust loss, blow-by loss, time loss, combustion efficiency, knock limit, combustion duration, combustion temperature and specific heat ratio.At a constant power output (324 kW) in a stoichiometric burn Otto cycle, the brake thermal efficiency was improved by 3 percentage points and the temperature of unburned gas in the cylinder was reduced by 36K when the engine speed was reduced from 1500 rpm (BMEP=1.12 MPa) to 1000 rpm (BMEP=1.68 MPa). The boost pressure and Pmax increased, but the Pmax did not exceed 8 MPa. Aside from Pmax, the temperature of the engine parts was also measured to evaluate the thermal and mechanical loads of the engine. The temperature of the exhaust gas, cylinder head and cylinder liner were reduced by 25K, 20K and 5K, respectively, but the piston temperature did not vary with engine speed. “Lower temperature and higher pressure combustion” was realized by reducing engine speed and increasing BMEP.When applying EGR and/or the Miller cycle to the stoichiometric burn natural gas engine (324 kW), lower engine speed (1000 rpm) specifications achieved a thermal efficiency of 37.6% in the Miller cycle without EGR; 37.9% in the Otto cycle with EGR; 38.4% in the Miller cycle with EGR. The thermal efficiency of the base engine (1500 rpm - 324 kW) at stoichiometric burn was 33.5%.